1. Field of the Invention
The present invention relates to a method for the operation of a power generation system utilizing a fuel cell, and more particularly to a method for controlling the fuel cell.
2. Description of the Prior Art
In a conventional generator system utilizing a fuel cell, hydrogen obtained by reforming a hydrocarbon is used as fuel. For example, when methanol is reformed with steam, hydrogen is obtained by the following reaction: EQU CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +3H.sub.2
When the above hydrogen-containing carbon dioxide and air are supplied to a phosphoric acid type fuel cell to generate electric power, the following reactions occur in an anode and a cathode of the fuel cell, respectively: EQU Anode: H.sub.2 .fwdarw.2H.sup.+ +2e EQU Cathode: 1/2O.sub.2 +2e+2H.sup.+ .fwdarw.H.sub.2 O
That is, the oxidation reaction of hydrogen proceeds in the anode, while the reduction reaction of oxygen in air as an oxidizing agent proceeds in the cathode, and water is obtained by an electrochemical reaction between hydrogen and oxygen as an entire reaction. In the course of this reaction, chemical energy is converted into electrical energy, which is taken out as an electric output to the exterior.
The current obtained by the above-mentioned reaction is proportional to the amounts of hydrogen and oxygen consumed according to Faraday's law.
When methanol is a starting material for obtaining hydrogen fuel, methanol and water are supplied to a reformer. In this case, the molar ratio of water to methanol supplied is about 1.3-2. Since water is supplied in an amount larger than the stoichiometrically calculated reaction amount as mentioned above, the reformed gas obtained by the reforming reaction contains water.
The reforming reaction of methanol is generally carried out at a temperature of about 250.degree. C. As a catalyst for the reforming reaction, a ZnO or CuO series catalyst is used and is filled in a reforming tube, through which a mixed vapor of methanol and water is passed to conduct the reforming reaction. Since the reforming reaction is endothermic, it is performed by heating the catalyst and the mixed vapor of methanol and water. The heat for this heating is obtained by supplying a fuel to a burner for the reformer and by burning it in air supplied from a fan for the reformer. In this case, the fuel can be supplied, for example, by the following three methods. The first method is a method of supplying methanol from a methanol tank through a methanol pump. The second method is a method of using an off-gas discharged from a fuel cell stack. The third method is a method of using the above methanol and off-gas together.
The mixed liquid of methanol and water supplied to the reformer evaporates in the reforming tube in the reformer. In the reforming tube, a reformed gas including hydrogen and carbon dioxide is produced by the action of the catalyst. This reformed gas is fed to a fuel gas chamber arranged on the side of a fuel electrode (anode) in the fuel cell stack. An excessive amount of off-gas including hydrogen, carbon dioxide and steam, which do not contribute to the electromotive reaction, is discharged from the fuel gas chamber and supplied to the burner for the reformer. As mentioned above, the combustion heat of the burner is the heat for accelerating the endothermic reaction.
The fuel cell stack is composed of a unit cell having a pair of electrodes; a fuel electrode (anode) and an air electrode (cathode). However, the output voltage of the unit cell having a pair of electrodes is about 1 V at most and the output current per unit area of the electrode is several hundreds mA/cm.sup.2. Therefore, a large output with a high voltage and a high current is obtained by using a plurality of stacks, each connecting a plurality of unit cells of large area in series and optionally, combining series and, parallel connections of these stacks.
The stoichiometric amounts of hydrogen and oxygen in air consumed by these stacks are proportional to the number n of unit cells in the stack and the output current I.
In general, the power generator system of the fuel cell is operated by supplying hydrogen and oxygen in amounts which are excessively larger than those theoretically consumed in the stack. The ratio of the consumption amount of each of hydrogen and oxygen to the supply amount of each of hydrogen and oxygen is called the utilization ratio. In the power generator system of the fuel cell, the hydrogen utilization ratio is 70-80%, and the oxygen utilization ratio (air utilization ratio in the case of air supply and consumption) is 50-60%.
The control of the hydrogen and oxygen utilization ratios is carried out by setting the output current from the stack and by supplying water and methanol as a reforming material to the reformer in proportion to the set current.
In the case of this control, if a time delay for applying water and methanol to the reformer to obtain a reformed gas and the reformed gas is supplied to the stack and the reforming reaction temperature are not properly controlled, the reforming reaction does not proceed sufficiently. Therefore, the output of the generator system utilizing the fuel cell should be controlled by considering the above-mentioned time delay. If such a control is not performed, hydrogen gas in the stack becomes short at the time that, for example, the operation of the fuel cell is started or the output is increased and hence the power generator cannot be driven.
Such a shortage of hydrogen gas results in a shortage of hydrogen gas in the off-gas and consequently, it may happen that the burner in the reformer misfires, and as a result the reformer is disabled.
Furthermore, in the case of a lighter gas shortage, which is less serious than the shortage of the hydrogen gas, gas shortage in the stack does not result, but the off-gas is short. As a result, the temperature of the reformer is lowered and the amount of the reformed gas decreases, and finally the operation of the power generator system utilizing the fuel cell is interrupted.
Moreover, the amount of the reformed gas becomes excessive when the output is lowered or when the output is interrupted, and hence it may happen that the amount of the off-gas becomes excessive and results in an increase in the temperature of the reformer.
In order to overcome the above drawbacks, the amount of reformed fuel gas has hitherto been controlled by a feedforward system or a feedback system in accordance with the output current from the fuel cell.
In the feedforward system, methanol as the fuel for the burner in the reformer is first burnt prior to the increase of the output current.
Then, the amounts of water and methanol as the starting material for the reformation are increased to increase the output current after the given delay time. In the case of decreasing the output current, there is adopted a method of first reducing the output current and then decreasing the output from the reformer, which is opposite to the case of increasing the output current.
On the other hand, in the feedback system, there is adopted a method of supplying an excessive amount of the reformed gas to the stack with consideration of the delay when the reformed gas is supplied.
In the conventional method for controlling the operation of the power generator system utilizing the fuel cell, however, an excessive amount of a fuel burnt in the burner for the reformer is required, so that energy is undesirably consumed. Furthermore, since the excessive fuel is burnt, the reformer is overheated and deteriorates the catalyst for the reforming reaction and, consequently there is the possibility that the reforming reaction does not proceed in a normal condition. Particularly, such drawbacks are notable in the case of the feedback system.